The operation of a centrifugal fan is much like that of a centrifugal pump with the pressure developed arising from two sources. There is a centrifugal force due to the rotation of an enclosed volume of air or gas, and a velocity imparted to the air or gas by the blades and partly converted to pressure by the volute or scroll-shaped fan casing.
The centrifugal force developed by the rotor produces a compression of the air or gas that, in fan engineering, is the static pressure. The amount of static pressure developed depends on the ratio of the velocity of the air leaving the tips of the blades to the velocity of the air entering the fan at the heel of the blades. Therefore, the longer the blades, the greater the static pressure developed by the fan. Operating efficiencies of fans are typically between 40 and 70 percent. Operating pressure is the sum of the static pressure and the velocity head of the air leaving the fan. It is generally expressed as inches of water gage, or in ounces per square inch.
The performance of a centrifugal fan varies with changes in conditions such as temperature, speed, and density of the gas. One significant operational problem with centrifugal fans is, that at higher operating conditions, a pulsation in pressure and flow within the centrifugal fan creates an audible beating noise. The cause of this is the creation of an unstable vortex that moves outside the impeller of the fan and into the inlet chamber of the fan.
Prior art solutions to this problem have included the addition of a radial splitter in the bottom of the inlet box. With this configuration, when the vortex escapes its confinement, it is immediately split by its impingement upon the splitter plate. Unfortunately, while this does destroy the vortex, it additionally destroys desirable pre-spin that was imparted to the inlet air by the positioning of the inlet dampers.
To date, there has been no effective solution to the audible pulsating that occurs when centrifugal fans operate in the "wide-open" position, without the attendant problems mentioned.